Epigenetic downregulation of the antibody and autoantibody response Epigenetic marks include DNA methylation, histone modifications and microRNAs. As we have contended, these interact with genetic programs to regulate B cell functions, such as class switch DNA recombination (CSR), somatic hypermutation (SHM) and plasma cell differentiation, thereby informing the antibody response. Epigenetic dysregulation can result in aberrant antibody responses to exogenous antigens, such as those on viruses and bacteria, or self-antigens, such as chromatin, histones and dsDNA in lupus. We hypothesize that the epigenetic modulators histone deacetylase (HDAC) inhibitors (HDIs) inhibit the B cell intrinsic functions CSR/SHM and plasma cell differentiation, thereby blunting antibody and autoantibody responses. We further argue that HDIs inhibit these B cell functions by upregulating selected microRNAs, including miR-155, miR-181b and miR-361 to downregulate AID (Aicda gene, critical for CSR/SHM), as well as miR-23b, miR-30a and miR-125 to downregulate Blimp1 (Prdm1 gene, critical for plasma cell differentiation). Our hypotheses uniquely focus on B cells and are supported by our compelling data using purified human and mouse B cells in vitro, and in vivo in normal mice responding to T-dependent and T- independent antigens, and prevention of disease and treatment of lupus-prone mice by HDI. Our strengths in B cell biology, molecular CSR/SHM mechanisms and autoimmunity, and our cutting-edge epigenetic tools and approaches make us uniquely poised to test our hypotheses. We will: (Aim 1) analyze the impact of HDIs (valproic acid, butyrate and SAHA) on CSR/SHM and plasma cell differentiation, and on specific T-dependent and T-independent antibody responses in normal C57BL/6 and Balb/c mice and autoantibody responses in lupus-prone MRL/Faslpr/lpr and B6.NZM/Sle1.Sle2.Sle3 mice; (Aim 2) use molecular biology, biochemistry and high-throughput/bioinformatics tools to analyze HDI-mediated upregulation of miR-155, miR-181b, miR-361, miR-23b, miR-30a and miR-125b in B cells through enhanced histone acetylation and transcription of the microRNA host genes, address downregulation of AID and Blimp1 by these microRNAs, and construct microRNA in vivo targeting maps of Aicda and Prdm1 3'UTRs using Ago HITS-CLIP; (Aim 3) prove the critical role of B cell microRNAs in mediating HDI suppression of antibody and autoantibody responses using a three-prong integrated in vivo approach involving construction of new knockin mice lacking specific microRNA targeting sites in Aicda or Prdm1 3'UTR, B cell conditional Dicer or Drosha KO mice, and mice expressing sponge inhibitors specific for miR-155, miR-181b and miR-361 (targeting Aicda) and/or for miR-23b, miR-30a and miR-125b (targeting Prdm1) in B cells. Our proposal is highly innovative and exquisitely translational. It will provide mechanistic insights and future directions in epigenetics and immunoregulation, including the critical role of B cell microRNAs in antibody/autoantibody responses and epigenetic tools as new therapeutics in autoimmunity.